Male Uber drivers earn around 7% more per hour than women, according to a new study that examined over a million Uber drivers in the US. Women were found to earn $1.24 per hour less than men, and also $130 less per week on average.

This finding runs counter to the assumption that the sharing economy is supposed to the great equaliser. Uber’s driver assignments and pay are supposed to be gender-blind, meaning a driver’s identity isn’t considered when matching riders or assigning fares. Rather, pay has to do with trip length, distance and whether it’s happening during surge-price hours or not.

The study, which was carried out by researchers at University of Chicago, Stanford University, and Uber’s own economic team and has not undergone peer review, attributed the difference in pay to fact that male Uber drivers:

Drive faster:According to Uber, on average, men drive 2.2% faster than women, and “there is a positive expected return to driving faster.”

Drive more: Drivers who have taken over 2,500 trips earn an average of $3 more per hour than those with less than 100 trips. Uber says, men, on average, accumulate more experience — and, hence, earn more — by working more hours each week and being less likely to stop driving with Uber.

Take on trips with shorter distances to the rider and chose to drive longer trips.

The issue of safety was not highlighted by the study though, which could play a key role in dissuading women from taking on higher paying rides. The MIT Technology review notes “Though it wasn’t covered in the study, one reason women may avoid higher-paying areas is that they don’t feel safe—they may opt not to drive late at night in certain places, for instance, or stay away from neighbourhoods that are considered dangerous.”

“Overall, our results suggest that, even in the gender-blind, transactional, flexible environment of the gig economy, gender-based preferences (especially the value of time not spent at paid work and, for drivers, preferences for driving speed) can open gender earnings gaps,” the study concludes.

Uber said the study produced “no evidence that outright discrimination, either by the app or by riders, is driving the gender earnings gap.”

Despite the variables causing this pay gap the results of the study might be an issue for the ride-hailing company. Uber has faced severe criticism for a toxic workplace culture that allowed male engineers to get away with sexually harassing their female colleagues.